The present invention relates to medium access control sublayer protocols for a shared medium, and more particularly, to repeaters and host interfaces that participate in a collision handling method of a medium access control sublayer protocol for a shared medium.
In any network in which multiple hosts are connected to a single shared communications channel (a multi-access system), there must be a method to resolve contention for the channel. In a multi-access system, data sent by one host will be received by all other hosts. If two different hosts attempt to transmit data simultaneously, there is a collision, and the data is lost. A transmission in which data has been lost is called a collision fragment. The various methods to resolve contests between the hosts and recover from collisions are called medium-access control (MAC) sublayer protocols.
Protocols in which multiple hosts listen for a carrier (i.e., a transmission) to determine whether the channel is currently busy are called carrier sense multiple-access (CSMA) protocols. By waiting until the channel is free to begin transmitting, some collisions are avoided. In an improvement over the CSMA protocol, the host ceases transmitting data to the channel if it detects a collision. Such protocols are called CSMA-CD protocols. Ending a transmission after a collision is detected conserves time and band-width. A host may detect a collision by comparing the data that it transmits to the signal it monitors on the channel.
The Institute of Electrical and Electronics Engineers (IEEE) has produced many standards for local area networks. One such local area network standard is the IEEE 802.3 CSMA-CD standard, originally developed from the Xerox Ethernet. The IEEE 802.3 standard is described in ANSI/IEEE Std.802.8, 1993 Edition, document no. ISO/IEC 18802-3:1993, the entirety of which is hereby incorporated by reference. The original Ethernet consisted of a passive bi-directional bus or cable segment, to which multiple host computers were connected by taps. Whenever a host transmitted data, the resulting signal was transmitted in both directions along the shared bus until the signal was absorbed by terminators at the ends of the bus. Hosts attached to the bus were able to read the signal as it passed by, whichever direction the signal was traveling. The original Ethernet is described in detail in U.S. Pat. No. 4,063,220, the entirety of which is hereby incorporated by reference.
Because of the limited length of its bus segments, the Ethernet design includes devices called repeaters to interconnect two or more buses. A repeater connected to more than two buses is referred to as a multiport repeater. Under the current IEEE 802.3 CSMA-CD standard, a repeater operates under fundamentally the same rules as the original Ethernet. Current repeaters transfer the signals arriving on each bus to any other bus to which the repeater is connected, so that larger and more complex networks may be built. In addition, repeaters "regenerate" the signals by retransmitting them at their original strength.
Under the MAC sublayer protocols of the current IEEE 802.3 CSMA-CD standard, collision handling is carried out entirely by the hosts. A host wishing to transmit data over a network must first wait for silence on the network before starting a transmission. Once the host begins transmitting, it checks for the arrival of other signals from the network which indicate that a collision is taking place. If the host detects the collision, it will (1) finish transmission of the preamble and start frame delimiter (SFD), if it has not already done so; (2) transmit a thirty-two bit jamming sequence; and (3) cease transmitting data. The host then waits until the network is silent (so that all collision fragments have been cleared) before initiating another transmission.
The time consumed in resolving a collision, i.e., the duration of the collision fragment plus the interframe gap, is unavailable for delivering valid data. In large networks a significant amount of time is wasted because a collision must propagate from the originating host to the end of the network and back.
In view of the foregoing, there is a need for an IEEE 802.3 CSMA-CD local area network device that reduces the time spent on collisions and increases the efficiency of the network.